Early processes for continuously deep frying potato slices and the like to produce potato chips utilized elongated cooking troughs or kettles containing a cooking oil which was heated by the direct contact of hot gases with the underside of the kettle bottom. Potato slices and other like materials to be processed were floated along the surface of the cooking oil and pushed throughout the length of the kettle by spaced-apart pusher assemblies.
More recent processes employed elongated troughs or kettles wherein the cooking oil was separately heated, either directly or indirectly e.g., a boiler or heat exchanger and passed to the cooking apparatus. A major portion of the oil was introduced into the kettle at the slice-receiving end of the kettle, withdrawn from the other end, reheated and re-introduced into the kettle. With due consideration to heat transfer requirements, i.e., sensible heat required to raise the potato slice to cooking temperatures, latent heat required to vaporize the water content of the slices, maximum temperature to which the cooking oil may be heated without excessive break down thereof to free fatty acids, etc., in view of the cooking time required for an average size potato slice, it was necessary to provide mechanical restraining devices, such as rotating flow wheels, rakes or submerger belts, intermediate the ends of the kettle to provide the necessary residence time for dewatering the potato slices (about 80% by weight of the slice) and for cooking the potato slices. This resulted in an oil flow rate through the kettle of about 5 to 6 times the rate of flow of the potato slices where a residence time of from about 3 to 4 minutes is required to produce an acceptable product. Final cooking of the slice is generally thought to be the reduction of the water content from about 10% to about 2% which is the moisture content of an acceptable product.
In many of the prior art apparatus, the potato slice had a tendency to adhere to the surface of the kettle as well as to the mechanical restraining devices in the early stage of cooking when the potato slice is in a pliable plastic state causing a dam effect thereby resulting in the development of areas of high cooking oil velocities in the clearance between the restraining mechanisms and the bottom and sides of the kettle. Some slices would surge through the areas of such high velocity and would not be retained in the kettle for a time sufficient to be properly cooked. Such high velocities of cooking oils created by the damming effect also had a tendency to develop eddies or whirlpools which would excessively restrain some of the slices being processed to the extent that the resulting chip was overcooked, and in some instances burnt thereby requiring manual removal prior to packaging. Breakage of an overcooked or burnt chip would result in fines in the cooking oil and fines in the cooking oil tend to break down the oil and form free fatty acids which are detrimental to the flavor and shelf life of the product. Additionally, high heat levels required at the inlet end necessitate high oil flow rates to provide for proper heat levels for final cooking of the slice.